1. Field of the Invention
The present invention relates to a coil of a local oscillator in a tuner, and more specifically to a fine tuning coil for adjusting the oscillation frequency of a local oscillator.
2. Prior Art
The local oscillation circuit of a tuner has been equipped with a plurality of oscillation coils to produce oscillation frequencies which correspond to a plurality of channels that will be received. In addition to these coils, there have also been provided coils for fine tuning to precisely adjust the oscillation frequencies. The coil for fine tuning usually consists of a copper wire of a diameter of about 0.5 mm coated with urethane, which is densely wound maintaining an inner diameter of about 5 mm. The coil is then solidified with an adhesive agent. The coil further movably accommodates a core member therein.
Fine tuning is effected by inserting the core member into, or taking it out from, the coil.
Usually, however, the coil for fine tuning consists of a covered wire which is densely wound, and, therefore, easily absorbs moisture among the neighboring wires. Therefore, stray capacitances often developed among the wires and these capacitances vary so that the coil oscillates at varying frequencies. It is therefore desired to provide a fine tuning coil having excellent characteristics against humidity. To prevent the characteristics of the fine tuning coil from being deteriorated by humidity, gaps between the wires may be increased to reduce the stray capacitances which principally stem from the absorbed moisture. This, however, means that the coil may not be as densely wound. To obtain the same inductance as that of the densely wound fine tuning coil, therefore, the size of the coil is inevitably increased. When not densely wound, furthermore, the coil tends to be easily affected by possible vibration when the tuner is subjected to vibration, giving rise to the occurrence of howling.